Phase locked loops are used in many diverse applications, including optical serial data links, multiple clock frequency generation, and FM demodulation. An important component of a phase locked loop is a voltage controlled oscillator (VCO). One type of VCO is a ring VCO which is typically composed of N stages of inverters with the outputs of the Nth stage being coupled to the inputs of the first stage. An external control voltage is also connected to each of the inverters to control the delay, t.sub.d of the inverters. The oscillation frequency of the circuit is determined by the number of stages N in the circuit and the bias current of the individual stages. Each stage has an oscillation period of 2t.sub.d. Thus a three stage ring oscillator has an oscillation period of 6t.sub.d.
Ring VCOs having fewer than three stages typically have severe reliability issues due to insufficient phase shift and incomplete switching. Another drawback of ring VCOs is the inability to decrease the oscillation period without decreasing the number of stages. This problem has become more critical as demands on integrated circuit performance have increased. The highest oscillation frequency which a VCO may achieve may be considerably less than the maximum frequency possible for the logic technology. Thus, it is desirable to provide a ring VCO design that has a reduced oscillation period which does not suffer from insufficient phase shift and incomplete switching.